Balanced hydraulic pump or motor of radial piston type



2,800,857 BALANCED HYDRAULIC PUMP 0R MOTOR OF RADIAL PISTON TYPE FiledSept. 22,1953. 1

J. YARMAK July. 30, 1951' July 30, 1957 J. YARMAK ,800,

BALANCED HYDRAULIC PUMP OR MOTOR OF RADIAL PISTON TYPE Filed Sept. 22,1953 5 Sheets-Sheet? y 1957 j J. YARMAK 2,800,857

BALANCED HYIDRAULIC PUMP OR MOTOR OF RADIAL PiSTON TYPE Filed Sept. 22,1953 5 Sheets-Sheet 5 k v l t Q L J y 1957 J. YARMAK 2,800,857

BALANCED HYDRAULIC PUMP OR MOTOR OF RADIAL PISTON TYPE Filed Sept. 22,1953 5 Sheets-Sheet 4 July 30, 1957 2,800,857

BALANCED HYDRAULIC PUMP 0R MOTOR OF RADIAL PISTON TYPE Filed Sept. 22.1953- J. YARMAK 5 Sheets-Sheet 5 Unite States Patent BALANCED HYDRAULICPUMP OR MOTOR RADIAL PISTON TYPE Julius Yarmalr, Lupset, Wakefield,England, assignor to Societe Stephanoise de Constructions Mecaniqucs,Salute-Etienne, France, a company of France Application September 22,1953, Serial No. 381,561

Claims priority, application France October 1, 1952 3 Claims. (Cl.103-3) characterised by application of rotating and reciprocatingmotions.

The invention has for its main object to provide a means for hydraulicand automatic balancing of internal forces arising in such hydraulicapparatus or transmission of power and which forces resist the variationand reversing of the delivery or supply. As is well known, the forcesmay, in some cases, be of a very considerable magnitude thusconstituting, if not balanced, an important obstacle to the properoperation of the apparatus.

According to one feature of the invention the means for providing acomplete balancing and neutralizing of such resisting forces may consistof several rocking hydraulic thrusters, each comprising a cylinder witha ram and communicated continuously one by one or in pairs, or in twogroups with the inlet and outlet ports respectively of th pump or motor.

The resisting forces, requiring balancing, comprise mainly an unbalancedresultant force or vectorial sum of all hydraulic forces created byliquid pressure acting on the inner ends of one part only of totalnumber of the radial pistons of such a hydraulic apparatus andparticularly the component force which is directed along the directionof displacement of the guide way for the radial pistons and of a secondcomponent force comprising friction on supports on which the guide wayfor the pistons thrust and reciprocate. Thus, the resisting forces in ahydraulic pump or motor, of the type concerned, depend on pressure ofworking liquids, on relative direction and on amount of eccentricity ordisplacement of the guide way for the radial pistons and on friction ontheir supports. All the above factors are not constant, therefore theresisting forces are changeable both in their magnitude and direction.

The balancing rocking cylinders or thrusters arranged and associatedwith a pump or motor according to this invention are adapted to provideforces alterable in the same way as the resisting forces of the pump ormotor and, having always a direction opposite to the direction of theresisting force are able to balance or neutralize the forces completely.

A balanced hydraulic pump or motor of the radial piston type and of avariable and reversible delivery, constructed in accordance with thisinvention, is particularly applicable in hydraulic transmissions adaptedfor continuously carrying out a quick reciprocating motion of a workingmember of an apparatus performed by means of a hydraulic servomotorconsisting of a driving piston movable in a cylinder such as is applied,for example, in the driving of a shaker conveyor troughing, forreciprocating screens, for planing and shaping machine tools and formany other machines requiring a continuous recipro- Patented July '30,1957 eating motion of various characteristics or for a quick reversiblerotating motion of other hydraulic motor.

Another object of the invention is to provide a means for a purelyrolling support of radial pistions of the pump or motor to preventhydraulic forces from acting on them and thus eliminating entirelysliding friction and avoiding loss of energy and wear.

Still another main object of the invention is to provide means ofoperation and such an arrangement of suction and relief valves of avariable and reversible delivery pump or motor as to enableself-adjusting of the reciprocating piston within the length of thecylinder and to prevent any possibility of fouling by the reciprocatingparts connected with the piston on the stationary cylinder or partsconnected with the cylinder of the hydraulic driving servomotor.

The above described main objects and features of the invention, as wellas other objects and features, will readily be understood from thedetailed description of an example embodying the invention where abalanced hydraulic pump of the radial piston type associated with a cammechanism is used and adapted for operation in a drive for shakerconveyors by a hydraulic transmission, which detailed description isgiven hereafter, referring to the accompanying drawings in which:

Figure 1 is a vertical longitudinal section through the central axle ofa balanced hydraulic pump, the section being taken along line 1-1 ofFigure 2.

Figure la is a continuation of Figure 1 to the right hand side.

Figure 2 is a vertical cross section along the line 2-2 of Figure 1.

Figure 3 is a cross section of the pump axle or pintle on an enlargedscale, taken along line 33 of Figure 1.

Figure 4 is a section along line 4-4 of Figure 1 showing arrangement ofcounter gears adapted for rotating the cams with required speed.

Figure 5 is a part section along vertical line 5-5 of Figure 2 showingboth cams and the operating screw.

Figure 6 is a part section along vertical line 6-6 of Figure 1 showingarrangement of valves and their operating means.

The balanced hydraulic pump of the radial piston type, with reference tothe drawings, comprises a cylinder block 11 with several-in this casefourteen--radial holes or cylinders 12 staggered or spaced in a zig-zagmanner and each housing a piston or plunger 13. The cylinder block 11 isrotatably mounted on a central immovable axle or pintle 14 by means of abush 15 pressed into the cylinder block, and is adapted to be revolvedby a coupling 16 and a shaft 17 (Figure la) rotatably mounted inanti-friction bearings 18 and connected with a shaft 19 of a drivingmotor 20 by means of cross coupling 21 comprising simultaneously aflywheel of a considerable moment of enertia to equalize the speed ofrotation of the motor and pump.

Each radial piston 13 is provided with a thrust roller or ring 22adapted to thrust at one side against a concave bearing surface 12a ofthe piston end inside the fork portion 13a and to bear at its oppositeside upon the inside cylindrical thrust surface of a drum 23 comprisinga race or a guide way for the pistons. The thrust roller or rings 22 areguided in grooves 11a provided in the cylinder block 11, and in the forkportion 13a of each piston 13, thus preventing the pistons from turningin their cylinder holes 12. Each thrust roller or ring 22 is linkedmovably to the fork portion of the outside piston end by means of anaxle pin 24 fixed to the fork portion 13a at the piston 13 and locatedin a central hole 24a of a relatively large .diameter provided in theroller 22.

The drum 23 is firmly attached by means of spigot portions and screws 25to two side rings 26 and 27. Said parts thus form together an outsiderotor of the pump or motor comprising the cylindrical guide way or racefor the pistons 13 and is rotatably supported by two roller bearings 28and 29.

It may be understood that during revolution of the cylinder block 11 bya driving motor with a usual high speed, the pistons 13'together withthe thrust rollers 22 will be pressed to the inside cylindrical surfaceof the drums 23 by centrifugal forces. It is obvious that in oneparticular position where the guide way or ra'ce drum 23 is concentricwith the axle 14 on which the cylinder block 11 revolves, all therollers 22 could be displaced very slightly only from the centre of theconcavity of the piston ends in their fork portions 1311 depending onthe direction of rotation so that all axle pins 24 will become eccentricto the same relative side of the holes 2411, and therefore the rotationof the cylinder block 11 will be transmitted to the guide way drum 23 byfriction forces between the thrust rollers 22 and the inside cylindricalguide surface of the drum 23. In this particular relative position ofthe guide way or race there will be neither any movement of the thrustrollers 22 relative to the pistons 13 nor any reciprocation of thepistons 13 relative to the cylinder block 11. All said parts will rotatearound the axle 14 with the same bodily speed without any slidingfriction between them and there will be no pumping action in thisparticular position.

It may be understood further that during rotation of the cylinder block11 in either direction and in the position where the guide way drum 23is displaced from the centre of the cylinder block 11, the pistons 13,being always forced out by centrifugal forces will follow thecylindrical eccentric guide way or race and, therefore, will perform onefull reciprocating movement during each revolution of the cylinder block11. During this process, the thrust rollers 22 will adjust themselvesrelative to the concave ends of the pistons 13 in the fork parts 13adepending on the amount of the relative eccentricity and direction ofthe rotation, so that the rollers 22 will roll slightly in bothdirections, i. e. rock or oscillate on the concave piston ends 12a inforks 13a during every revolution of the cylinder block 11 to compensatein this way for any change of distance between the piston ends and anydifference in relative peripheral speed of rotation repeated duringevery revolution when following the inside contour of the eccentricguide way or race during the pump or motor operation.

The guide way drum 23 will be rotated with an average and constant speedby that part only of the rollers 22 which in turn are under pressure andby the tangential and friction forces in similar manner to thatdescribed above for the concentric position. The tangential forces willdepend of course on the amount of displacement of the rollers relativeto the piston ends. It may be noticed that there will be only rollingand no sliding friction between the concave end at the pistons 13, therollers 22 and the drum 23, provided the clearance for the axle pin 24in the hole 24a is adequate to cover the maximum amount of eccentricityof the guide ways for the pistons. Both roller bearings 28 and 29 aremounted on integral collars of two side plates 30 and 31 respectively,of a rigid cradle frame which includes also two curved plates 32 and 33(Figure 2). All said parts, forming the reciprocating cradle frame, aresecured together by means of screws 34 and dowel pins 34a. 7

The cradle frame, as a whole, is slidably supported by means of twopairs of pads 35 and 36. The pads 35 are thrusting on and sliding acrossthe central axle or pintle 14 and the pads 36 are thrusting on andsliding across a spigot bush 37. The curved plate 33 of the cradle frameis provided with an integral rigid tail piece 38 slidably fitted in bush39 of a spigotflange 40 attached to side wall of housing. Therefore, thecradle frame, being supported'at three points 35, 36 and 39, is,able toreciprocate in a horizontal plane across the pump axle 14 todis- 4 placethe centre 41 of the guide way or race circle for the pistons 13 in bothdirections relatively to the centre 42 of the axle 14, which is also thecentre of rotation for the cylinder block 11.

During revolving of the cylinder block 11, each piston 13, being alwayspressed to the inside cylindrical surface of the guide way drum 23 bycentrifugal forces, will perform one full reciprocating movement in itscylinder hole 12 during every revolution of the cylinder block 11, andthe amplitude or length of stroke of pistons 13 is always equal to thedouble eccentricity or double distance between two centres 41 and 42.

Inside the axle or pintle 14, comprising simultaneously a rotary typevalve of the pump, as is usual for pumps of this kind, two pairs ofholes 43 and 44 are provided. Holes 43 are both communicated with theflexible pipe 45 and the holes 44 are both communicated with theflexible pipe 46. Flexible pipes 45 and 46 lead to opposite sides of adriving piston in a cylinder forming a hydraulic servomotor (not shownin the drawings). Both pipes 45 and 46 may be either inlet or outletports of the pump or motor alternatively depending upon the direction ofthe eccentricity by a constant direction of the cylinder block rotation.

For a continuous alteration of the eccentricity and thus the quantityand direction of working liquid supply in one particular applicationcase of a balanced hydraulic pump or motor, constructed according to thepresent invention, a cam mechanism is employed comprising a rotatableelongated disc cam 47, slidably mounted on a shaft 48 which is rotatedwith a required speed from the driving shaft 17 (Figure 4) by means ofgears 49, 50, 51, 52, 53 and 54 arranged inside a housing 55. The cam 47is adapted to act on a roller 56 rotatably mounted in a projectingportion of the curved plate 32 of the cradle frame. A compression spring57 is adapted to force continuously the whole cradle frame with theroller bearings 28 and 29 and the outside rotor of the pump includingguide w'ay drum 23, against the cam 47 to perform a full reciprocationand therefore a full cycle of supply alteration of the pump during everyfull revolution of the cam 47.

The cam 47 comprises an elongated body of a sloping cylindrically curvedshape changeable along its axis so that any cross section comprises itsown working profile of a particular shape, and may be slid along itsshaft 48 by means of a screw 58 provided with both right hand and lefthand thread parts and mounted in a'threaded bush 59 fixed at the wall ofthe outside housing. When rotated by means of a handle 60, the screw 53will move itself, moving with double speed a threaded nut integral witha fork 61 engaging the collar part 62 and thus sliding the cam 47 ineither direction along the shaft 48, depending on the direction ofrotation of the screw 58 and changing simultaneously the actual workingprofile of the cam 47 which is in contact with the roller 56. In thisway a characteristic of reciprocating motion may be easily altered.

The importance of all forces balancing for an easy and unrestrictedmovement of the cradle frame, carrying the guide way or race drum 23 inthe above described cam mechanism, may be well appreciated when takinginto account that the cradle frame must be in a continuousreciprocation, being under the influence of the hydraulic forces actingon one part only of the pistons 13.

Although any direct thrust from the cylinder block 11 through the bush15 upon the'axle 14 may be completely neutralized hydraulically byprovision of a correct relation between the size of holes in the bush1'5 and the grooves in the axle 14 so that there will be no pressure,therefore little friction and wear of these two parts. Nevertheless, thereactions to the hydraulic forces, causing the thrust of the rollers 22at the outside ends of pistons 13 from their guide way or race, willcreate a very considerable vertical force because only a half of theto-' tal number of pistons 13 (i. e. only those which are either aboveor below the central horizontal plane) may be under pressure. Thecreated force is transferred through the bearings 28 and 29 to theplates 30 and 31 of the cradle frame and could cause, if not balanced, aconsiderable pressure, friction and resisting force between the pads 35and 36 and their supporting means. It may be found that there existsalso a horizontal force, that is, a force acting along the reciprocationdirection of the cradle frame with the guide way drum 23. The forcearises because of the eccentricity of the guide way or race relativelyto the rotation centre of the cylinder block 11. It may be noticed thata relatively larger part of the cylinder block 11, representingtherefore a relatively larger number of the radial pistons 13 underpressure, acts to resist an increase in the eccentricity of the guideway or race than those acting to assist the increase.

For a balancing of all the above described forces, there are providedfour identical rocking hydraulic thrusters 63, 64, 65 and 66 (on Figure2 one thruster 63 is shown in section and the rest are shown in outsideview). Each thruster comprises a cylinder 67 with a wedge shaped thrustblade 68 and a piston or plunger 69 with its similar wedge shaped thrustblade 70. For tightness there is provided a piston ring 71. A lightspring 72 is adapted to assure continuous contact between the blades 68and 70 and their thrust pads 73, 73a, 74 and 74a in cases wherethethruster is not under pressure. Two thrusting pads 74 are mountedfirmly in a projecting bracket 75 forming an integral part of the curvedplate 32 and bearing the thrust roller 56. Two other thrust pads 74a aremounted firmly in the tail piece 38 integral with the curved plate 33.Two thrust pads 73 are firmly and immovably mounted in beams 76 and twolast thrust pads 73a are mounted immovably at the bottom of the housing.

Both upper thrusters 63 and 65 are communicated continuously in a firstpair with the upper bores 43 by means of two flexible pipes 77 and 77a(Figures 2 and 6) and both lower thrusters 64 and 66 are communicatedcontinuously in another pair with the bores 44 by means of two otherflexible pipes 78 and 78a.

It may be understood from the above description that, under influence ofthe cam 47 and the spring 57, the cradle frame together with brackets 75and tail piece 38 will perform a reciprocating movement so that centre41 will be displaced alternatively to the left and to the right side inrelation to the centre 42 and all four thrusters 63, 64, 65 and 66 willrock simultaneously. The relative positions of the immovable thrust pads73 and 74 are such that when the centres 41 and 42 are in the same point(i. e. when there is no eccentricity) all thrusters are in vertical orneutral positions. The diameter of the truster pistons 69 is of such asize that each pair of thrusters 63 and 65 or 64 and 66 will be able tobalance the vertical component force of all hydraulic forces acting onpistons 13, and the length of the thrusters is such that each pair ofthem will be able to produce, when tilted from the vertical position, aforce directed along the direction of eccentricity and of the samemagnitude and increasing in the same proportion as the horizontal forceresisting the displacement of the cradle frame, thus being able tobalance the resisting force.

By the arrangement described above, all forces resisting the movement ofthe cradle frame will always be balanced hydraulically because as may beunderstood, by either direction of rotation of the cylinder block 11, byany pressure of the working liquid in either bore 43 or 44 (or in bothbores simultaneously) and by any eccentricity of the guide way forpistons 13, the rocking hydraulic trusters will simultaneously andautomatically provide balancing forces or counterpressures acting on thereciprocating cradle frame supporting the race which forces are equal inamount and opposite in direction to the forces resisting thedisplacement. Therefore, the required reciprocation of the cradle framefor variation and 6 change in direction of the pump supply may beperformed easily and unrestrictedly.

To provide means of self-adjustment for the reciprocating driving pistonwithin the cylinder length of the servomoter, adapted for reciprocatinga shaker conveyor troughing or similar apparatus members of aconsiderable momentum by means of allowing a free slip of the hydraulicliquid excess which may occur on either side of the driving pistonbecause of leakage and other casual reasons, in a balanced hydraulicpump constructed according to the invention, a rocking lever mechanismadapted for lifting suction valves synchroneously with every reverse ofthe hydraulic servomotor stroke is arranged.

The lever mechanism is associated with the common suction and reliefvalves which are provided as usual in hydraulic apparatus of such a kindand are adapted to replace any leakage and other loss of liquid in thesystem and to act as safety means, preventing over-pressure.

In this particular case, a separate suction valve 80 or 80a with a ball81 or 81a and a relief valve 82 with a ball 83 loaded by spring 84regulated by a threaded plug 85 is associated with each bore 43 or 44.

Both valves 80 and 82, as may be seen on the drawing (Figure 6), areconnected with the bore 44 and are both included in a separable valveblock 86 that is fluid tightly secured to the axle 14 by two screws 87.Fully identical suction and relief valves are connected with the secondbores 43 of the axle 14. Both last valves are included in the secondseparable valve block 89 secured fluid tight to the opposite side of theaxle 14 by means of two screws 90 To lift either suction valve 80 or 80aso as to keep them open during required periods of the servomotor pistonstroke and each time just when direction of the piston movement isaltered, two thrust screws or tappets 91 and 92 securely mounted in arocking lever 93 are provided and are adapted for a rocking movementwith an axle 94 (Figure 6). The rocking lever 93 is connected by a bar95 and a thrust spring 96 with a second lever 97 rocking with itssupporting axle 98 by means of a weight 99 and roller 100 operated by aneccentric disc cam 101 securely mounted on the same shaft 48 where theelongated cam 47 adapted for variation and change in direction ofpumping is slidably mounted. The disc cam 101 (Figure 6) is provided onits working profile with two cylindrical portions 102 and 103 of such aradius as to keep both thrust screws or tappets 91 and 92 in the neutralposition without reaching either of two valve balls 81 and 81a. A raisedportion 104 of the disc cam is adapted to lift and keep open the valve80 during a period when the driving piston of the servomotor is changingthe direction of its movement. A lowered portion 105' of the disc cam isadapted to lift and keep open the second valve 8011 when the drivingpiston is changing its direction of movement again. It may be understoodthat the suction valve, thus lifted and kept open, will prevent buildingup counterpressure by permitting a free escape of the hydraulic liquidfrom the corresponding side of the servomotor cylinder when the drivingpiston is changing its direction of movement and is pushed and thusadjusted nearer to the central position in the cylinder under theinfluence of pressure springs or other cushion means usually provided ona servomotor of that kind.

The described hydraulic pump or motor is housed inside an enclosedcasing 107 provided with a cover 108 and mounted on a base plate 109 onwhich the driving motor 20 is mounted as well. The inside space of thecasing 107 is divided into two chambers or compartments 110 and 111 by avertical wall 112. The chamber 111 comprises the hydraulic liquidcontainer having a high liquid level 113. As may be seen in figure 1,the central axle 14 passes through the chamber 111 from an outside wall114 to the wall 112 by which walls the axle 14 is firmly supported. Bothvalve blocks 86 and 89, including the suction and relief valves are,located in the chamber 111. The rotating parts of the pump or motor are1'0 cated in the chamber 110 having a low liquid level 115. Thus,provisions are madetfor rotation of the pump parts, not in an oil bath,but in air, enabling a high speed of rotation because the frictionbetween oil and rotating parts of pump, which friction at the high speedwould be very considerable, is eliminated. I

To keep the liquid level 115 in the room 110 at the 're quired lowheight, it will be necessary to remove any exeess liquid which may comefrom leakage during the pump operation. For that purpose, a downwardlysloping oil reflect-or 116, in the form of a corrugated sheet, isarranged at the top of the housing above the pump rotor, By thisarrangement, any oil in excess of the level 115 will be caught byimmerse'drotating parts of the pump rotor, including the drum 23, anddropped continuously on the corrugated sheet or oil reflector 116 to letthe excess oil flow freely back into the container 111. It may beunderstood that all parts inside the housing 107 will be well lubricatedby oil mist and drops created by the quick rotating parts of the pump.

For the purpose of oil filling and the level inspection, there isprovided a small cover 117 with a threaded plug 118 and on oil strainer119.

The small housing 55, containing counter gears 49, 50, 51, 52, 53 and54, is separated from the room 110 by an oil seal 120 and is secured tothe housing 107 by four screws 121 and is provided with its inspectioncover 122 held in position by screws 123.

It may be well understood that the pump described above will act as amotor when the hydraulic liquid will be supplied under pressure througheither one of the pipes 45 or 46. Therefore, during that part of everyreciprocation of the conveyor troughing (or other reciprocating parts ofan apparatus) or that part of the stroke during which the troughing isaccelerated, the above described hydraulic mechanism will work as apump; supplying working liquid under pressure and transmitting theenergy of the driving motor 20 and partly the kinetic energy accumulatedin the speedy rotating flywheel 21 to the reciprocating troughing (orother parts), but during the next part of the conveyor stroke comprisingretardation of the troughing, the hydraulic mechanism will act as amotor using working liquid under pressure and supplying energy to theflywheel and driving motor accelerating them.

It may be understood as well that in both cases when the hydraulicmechanism in acting either as a pump or as a motor, the function of thebalancing arrangement as described above will be performed equallysmoothly and the result of balancing will be equally important.

What I claim is:

1. In a hydraulic reversible pump of the radial piston type, whichincludes a cylindrical casing, acylinder block rotatably disposed in thecasing and provided with radial cylinders'and pistons slidable in saidcylinders; meansfor a purely rolling support of the radial pistonsagainst hydraulic, centrifugal and inertia forces to which thepistonsare subjected during operation of the pump, said means comprising abifurcation provided at the outer end of each of said radial pistons, aconcave cylindrical surface formed on each end of the pistons betweenthe branches of the bifurcations, said cylindrical surface havingitsaxis parallel to the axis of rotation of the pump, a thrust rollerlocated between the branches of each bifurcation, each roller beinginterposed by opposite sides of its outer periphery between the concavesurface of the pistons and the inner cylindrical guide surface of thecasing, said casing being displacable eccentrically and freelyrotatable, an axle pin secured in the branches of each bifurcation, eachroller having a central hole re ceiving the pin, each of said pins beingof smaller diameter than the holes to provide a movable linkage of eachroller in -each bifurcation thus enabling a limited oscillating rollingmovement for each roller along the concave cylindrical surface of eachpiston and along the guide surface of the casing.

2. A hydraulic reversible pump of the radial piston type comprising ahousing having outer vertical walls and an inner vertical wall, saidlast wall dividing the housing into two oil tight compartments, a pintlehorizontally mounted in one of the outer walls and in the inner wall andpassing entirely through one of the compartments and projecting into theother compartment, said pintle being provided with an upper and a lowerduct, the ducts selectively serving as suction and exhaust ductsaccording to the direction of rotation of the pump, a cylinder blockrotatably supported on the projecting end of the pintle in the othercompartment, a horizontal shaft coupled to the cylinder block forrotating the block, radial cylinders provided in the cylinder block,pistons reciprocating in said cylinders, a reciprocating cradle disposedin said other compartment and mounted for movement transversely to theaxis of the pintle, a guide way drum rotatably supported on the cradlefor free rotation thereon and having an inner cylindrical guide surface,roller means on the outer ends of the pistons and disposed in rollingcontact with the guide surface, means for continuously reciprocating thecradle to displace the drum eccentrically in two opposite directionswith respect to said cylinder block for modifying the stroke of saidpistons in the cylinders, two sets of balancing rocking hydraulicthrusters, one of said sets being disposed above the horizontal planecontaining the axis of the pintle and the second set being disposedbelow said plane and between said housing and said cradle and adapted toprovide forces directed substantially at right angles with respect tothe direction of reciprocation of said cradle, each of said thrustersincluding a cylinder having an open end and a knife edged closed end, apiston sealingly and slidingly fitted in each of said thruster cylindersand having a knife edged outer end, a compression spring disposed insideeach thruster cylinder and bearing against the closed end of thecylinder and the inner end of the piston to extend the thruster piston,a flexible pipe connected to each thruster cylinder, the flexible pipesleading to the set of thrusters disposed above the, horizontal plane ofthe pintle being connected with the upper duct of the pintle and theflexible pipes leading to the thrusters of the set of thrusters disposedbelow said horizontal plane being connected with the lower duct in saidpintle.

3. A hydraulic reversible pump as claimed in claim 2, wherein a plate isdisposed above said guide way drum in a sloping position from the othercompartment towards the one compartment so that any leakage of oil fromthe pump in the other compartment is caught by' the drum andcentrifugally thrown on said plate and returned thereby into the onecompartment.

References Cited in the file of this patent UNITED STATES PATENTS1,628,603 Ferris May 10, 1927 2,107,079 Mentele Feb. 1, 1938 2,420,806Anderson May 20, 1947 2,427,224 Morton Sept. 9, 1947 2,458,197 PierceJan. 4, 1949 2,502,546 Adams Apr. 4, 1950 2,539,571 Deschamps Jan. 30,1951 2,585,232 Douglas Feb. 12, 1952 2,614,500 Tucker Oct. 21, 19522,662,635 Joy Dec. 15 1953 2,678,607 Hufierd May- 18, 1954

